Photomask inspection method and apparatus thereof

ABSTRACT

A method includes: receiving a photomask; patterning a wafer by directing a first radiation beam to the wafer through the photomask at a first tilt angle; and inspecting the photomask. The inspecting includes: directing a second radiation beam to the photomask at a second tilt angle greater than the first tilt angle; receiving a third radiation beam reflected from the photomask; and generating an image of the photomask according to the third radiation beam.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims priority to U.S. Provisional Application No.63/075,584 filed Sep. 8, 2020, the disclosure of which is herebyincorporated by reference in its entirety.

BACKGROUND

In advanced semiconductor technologies, the continuing reduction indevice size and increasingly complex circuit arrangements have made thedesign and fabrication of integrated circuits (ICs) more challenging andcostly. To pursue better device performance with smaller footprint andless power consumption, advanced lithography technologies, e.g., extremeultraviolet (EUV) lithography, have been investigated as approaches tomanufacturing semiconductor devices with a relatively small line width,e.g., 30 nm or less. EUV lithography employs a photomask to control theirradiation of a substrate under EUV radiation so as to form a patternon the substrate.

While existing lithography techniques have improved, they still fail tomeet requirements in many aspects. For example, there is a need toimprove the quality of the photomask image in a photomask inspectionoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a schematic diagram of an inspection apparatus, in accordancewith some embodiments of the present disclosure.

FIG. 2 is a schematic cross-sectional view of a photomask, in accordancewith some embodiments of the present disclosure.

FIG. 3A is a schematic top view of a projection optics box, inaccordance with some embodiments of the present disclosure.

FIG. 3B is a schematic view of an incident radiation beam and areflection radiation beam in an inspection operation, in accordance withsome embodiments of the present disclosure.

FIG. 4A is a schematic top view of a projection optics box, inaccordance with some embodiments of the present disclosure.

FIG. 4B is a schematic top view of a projection optics box, inaccordance with some embodiments of the present disclosure.

FIG. 4C is a schematic top view of a projection optics box, inaccordance with some embodiments of the present disclosure.

FIG. 5 is a schematic diagram of a lithography apparatus, in accordancewith some embodiments of the present disclosure.

FIG. 6 is a flowchart of a method of inspecting a photomask, inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 70 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the deviation normally found in therespective testing measurements. Also, as used herein, the terms“about,” “substantial” or “substantially” generally mean within 10%, 5%,1% or 0.5% of a given value or range. Alternatively, the terms “about,”“substantial” or “substantially” mean within an acceptable standarderror of the mean when considered by one of ordinary skill in the art.Other than in the operating/working examples, or unless otherwiseexpressly specified, all of the numerical ranges, amounts, values andpercentages such as those for quantities of materials, durations oftimes, temperatures, operating conditions, ratios of amounts, and thelikes thereof disclosed herein should be understood as modified in allinstances by the terms “about,” “substantial” or “substantially.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the present disclosure and attached claims areapproximations that can vary as desired. At the very least, eachnumerical parameter should at least be construed in light of the numberof reported significant digits and by applying ordinary roundingtechniques. Ranges can be expressed herein as being from one endpoint toanother endpoint or between two endpoints. All ranges disclosed hereinare inclusive of the endpoints, unless specified otherwise.

The terms “photomask,” “reticle” and “mask” used throughout the presentdisclosure refer to a device used in a lithography system, in which apatterned image according to a circuit pattern is formed on a substrateplate. The substrate plate may be transparent. The image of the circuitpattern on the photomask is transferred to a workpiece through aradiation source of the lithography system. Lithography radiationemitted from the radiation source is incident on the workpiece via thephotomask in a transmissive or reflective manner.

The advanced lithography process, method, and materials described in thecurrent disclosure can be used in many applications, including fin-typefield effect transistors (FinFETs). For example, the fins may bepatterned to produce a relatively close spacing between features, forwhich the above disclosure is well suited. In addition, spacers used informing fins of FinFETs can be processed according to the abovedisclosure.

An extreme ultraviolet (EUV) photomask is typically a reflective maskthat includes a circuit pattern formed thereon and is used to transferthe circuit pattern onto a workpiece, such as a wafer, throughreflection of a patterned EUV radiation from a light-reflective layer ofthe EUV photomask during a lithography operation. The EUV photomaskgenerally includes an anti-reflection coating (ARC) and alight-absorption layer (LAL) above the light-reflective layer, in whichthe ARC and the LAL are patterned to form the circuit pattern. Thepatterned EUV light is reflected from the light-reflective layer,through the patterned LAL and the ARC, and radiated onto the wafer.

After an EUV photomask is fabricated or when a fabricated EUV photomaskhas been operated for predetermined period, a routine photomaskinspection is performed to ensure integrity and performance of the EUVphotomask. If a defect or undesirable material is found in the EUVphotomask, for example, if a phase defect is found in thelight-reflective layer or a foreign particle is formed in the ARC or theLAL, a repairing operation is required to fix the defect. The inspectionis generally performed by scanning the photomask to generate an imageand examining whether any defect is found in the image. In advancedtechnology generations, the photomask defects have become smaller andmore difficult to identify than those in previous technologygenerations. As a result, greater resolution of the inspection image isrequired to guarantee identification of all defects and provide asufficient process window of a patterning operation.

The present disclosure provides a method of increasing quality andresolution of an inspection image of an EUV photomask. In the proposedscheme, an inspection apparatus is modified, in which an aperture forfiltering inspection radiation beam is redesigned to have a greaterdiameter or width with a symmetrical shape to increase radiationintensity while reducing image distortion. In addition, tilt angle(referred to as a chief ray angle) of the incident radiation impingingonto the photomask is tuned according to the aperture, e.g., tuned to begreater than a tilt angle of a radiation beam used in a patteringoperation, for improving light collection performance. Therefore, theproposed scheme generates an inspection image with greater intensityuniformity and reduced image distortion compared to images generated byexisting inspection apparatuses. The effectiveness of defect detectionis significantly enhanced accordingly.

FIG. 1 is a schematic diagram of an inspection apparatus 100, inaccordance with some embodiments of the present disclosure. In someembodiments, the inspection apparatus 100 is an EUV photomask inspectionapparatus. In some embodiments, the inspection apparatus 100 is areflection type inspection apparatus, a transmission type inspectionapparatus, or a combination thereof. As shown in FIG. 1 , the inspectionapparatus 100 includes a radiation source 102 and a chamber 104. In someembodiments, the inspection apparatus 100 includes a channel 103connecting the radiation source 102 and the chamber 104. In someembodiments, the radiation source 102 and the channel 103 are integratedinto the chamber 104. In some embodiments, additional modules of theinspection apparatus 100, e.g., a power supply and a control device, maybe present but are omitted from FIG. 1 for brevity.

The radiation source 102 is configured to generate a source radiationbeam R0 and emit the source radiation beam R0 into the chamber 104through the channel 103. In some embodiments, the source radiation beamR0 has a wavelength between about 1 nm and about 100 nm, such as about13.5 nm. The source radiation beam R0 may be EUV light. In some otherembodiments, the source radiation beam R0 has a wavelength of deep UV(DUV) or another suitable wavelength. In some embodiments, the radiationsource 102 includes a plasma source, such as discharge-produced plasma(DPP) or laser-produced plasma (LPP). In some embodiments, the radiationsource 102 also includes a collector, which may be used to collect lightgenerated from the plasma source and to emit the source radiation beamR0 toward the chamber 104.

In some embodiments, the chamber 104 includes an illuminator(illumination system) 106, a mirror 108, a stage 110, a projectionoptics box (POB) 120, a detector 130, and a processor 140.

The illuminator 106 is configured to receive the source radiation beamR0 through the channel 103 to generate a radiation beam R1. In someembodiments, the illuminator 106 includes reflective optics, such as oneor more mirrors, to direct light from the radiation source 102 onto themirror 108 or the stage 110. In some examples, the illuminator 106 mayinclude a zone plate, for example, to improve focus of the sourceradiation beam R0. In some embodiments, the illuminator 106 isconfigured to shape the source radiation beam R0 into, for example, adipole shape, a quadrapole shape, an annular shape, a single beam shape,a multiple beam shape, and/or a combination thereof.

In some embodiments, the illuminator 106 includes, but is not limitedto, an adjuster, an integrator and a condenser. In some embodiments, theilluminator 106 is configured to condition the source radiation beam R0according to predetermined specifications, such as intensitydistribution and uniformity of the radiation beam R1. Iii someembodiments, illuminator 106 adjusts the angular intensity distributionof the source radiation beam R0.

The radiation beam R1 is incident on the mirror 108, reflected by themirror 108 and impinges onto the stage 110. The radiation beam impingingon the stage 110 is referred to as an illumination radiation beam R2. Insome embodiments, the mirror 108 is formed of materials having an EUVreflectivity of greater than about 50%. In some embodiments, the mirror108 has an EUV reflectivity of greater than 60% or greater than about80%. The mirror 108 may include a multilayer structure. The mirror 108may include pairs of light-reflective layers, wherein each pair is,e.g., formed of a molybdenum layer and a silicon layer. The number ofalternating molybdenum layers and silicon layers (i.e., the number ofmolybdenum/silicon pairs) and the thicknesses of the molybdenum layersand the silicon layers are determined so as to facilitate constructiveinterference of individual reflected rays (referred to as Braggreflection) to thereby increase the reflectivity of the mirror 108. Themirror 108 may be a planar mirror or an ellipsoidal mirror. In someembodiments, an incident (tilt) angle of the radiation beam R2 onto thephotomask 111 is controlled by a tilt angle of the mirror 108.

In some embodiments, the chamber 104 includes one or more lenses ormirrors in the optical path between the radiation source 102 and thestage 110 to process or direct the radiation beam R0, R1 or R2, Forexample, an optical filter may be utilized to filter unwantedwavelengths of the source radiation beam R0. In some other examples, oneor more ellipsoidal mirrors are provided in the chamber 104 to reflectand direct the source radiation beam R0 toward the mirror 108. Theellipsoidal mirror may include a multilayer structure made of molybdenumand silicon.

During an inspection operation, a workpiece, e.g., a photomask 111, isprovided for inspection. In some embodiments, the stage 110 is used forsupporting and holding the workpiece. The stage 110 may include one ormore positioning devices, such as motors and roller guides, to provideaccurate alignment and movement of the workpiece in various directionsfor achieving better performance in focusing, leveling, exposure orother movements.

In some embodiments, a pump unit (not separately shown) is configured toprovide a substantially vacuum or high vacuum environment of the chamber104 of the inspection apparatus 100. In some embodiments, the pump unitincludes one or more pumps and filters.

The photomask 111 reflects the radiation beam R2 to form a reflectedradiation beam R3. The radiation beam R3 is directed into the POB 120.During an inspection operation, the surface of the photomask 111 may bepartitioned into a grid array and the radiation beam R2 is narrowed toilluminate each grid successively to complete the scan loop. When aphase defect or a surface foreign substance having a sufficient sizeexists in a grid where the radiation beam R2 is collected on thephotomask 111, the radiation beam R3 will be scattered by the defect andthe rays of the radiation beam R3 may go in different directions. Theinformation of the defect is carried by the radiation beam R3 when theradiation beam R3 travels through the POB 120 and is detected by thedetector 130.

In some embodiments, the POB 120 includes an aperture stop 122, a firstreflective element 124, a second reflective element 126 and a holder128. The aperture stop 122 is configured to filter the radiation beam R3by blocking portions of the radiation beam R3, In some embodiments, theaperture stop 122 include an aperture 122A configured to allow desirableportions of the radiation beam R3 to pass through. In some embodiments,the holder 128 is an optical element holder used to hold and secure thefirst reflective element 124 and the aperture stop 122. The secondreflective element 126 is arranged below the first reflective element124 and may be hanged by one more support arms (not shown), in which thesupport arms are formed of rigid materials and extend from the aperturestop 122 or the holder 128 to the second reflective element 126.

In some embodiments, the holder 128 has a circular or ring shape from atop-view perspective, as illustrated in FIG. 3A. In some embodiments,the first reflective element 124 is laterally surrounded by the holder128. In some embodiments, the aperture stop 122 is coupled to the holder128 and integrated with the holder 128. In some embodiments, theaperture stop 122 is part of the holder 128, in which a first part ofthe holder 128 has a ring shape laterally surrounding the firstreflective element 124 and a second part (corresponding to the aperturestop 122) of the holder 128 is coupled to the first part and has acircular shape defining an opening allowing the radiation beam R3 topass through and reach the first reflective element 124. In someembodiments, the second part of the holder 128 is immediately betweenthe first reflective element 124 and the second reflective element 126.

In some embodiments, the reflective elements 124 and 126 are configuredto form a Schwarzschild illumination system. The first reflectiveelement 124 may be configured as a condenser to reflect the radiationbeam R3, in which a projected radiation beam R4 is collected by thesecond reflective element 126 to form a projected radiation beam R5,which travels through an opening of the reflective element 124 towardthe detector 130. The Schwarzschild illumination system permits controlof the collection angle of the radiation beam R5, which is intended tobe received by the detector 130. In some embodiments, the reflectiveelement 124 or 126 includes a planar mirror or an ellipsoidal mirror.

In some embodiments, the detector 130 is configured to generate aninspection image according to the received radiation beam R5. In someembodiments, the detector 130 is a photodetector, such as a solid stateimage sensor, e.g., a CCD or CMOS image sensor. The processor 140 mayinclude a processing unit configured to generate the inspection imageaccording to the electrical signals provided by the detector 130. Insome embodiments, the processor 140 is configured to performcomputations to identify whether a defect exists.

FIG. 2 is a schematic cross-sectional view of a photomask 200, inaccordance with some embodiments of the present disclosure. Thephotomask 200 may be used as the photomask 111 as illustrated in FIG. 1. Referring to FIG. 2 , the structure of the photomask 200 includes asubstrate 202, a multilayer stack 204, a capping layer 206 and alight-absorption layer 208. Other configurations of the photomask 200,such as additional layers, may also be within the contemplated scope ofthe present disclosure.

The substrate 202 is formed of a low thermal expansion (LTE) material,such as fused silica, fused quartz, silicon, silicon carbide, blackdiamond and other low thermal expansion substances. In some embodiments,the substrate 202 serves to reduce image distortion resulting from maskheating. In the present embodiment, the substrate 202 includes materialproperties of a low defect level and a smooth surface. In someembodiments, the substrate 202 transmits light at a predeterminedspectrum, such as visible wavelengths, infrared wavelengths near thevisible spectrum (near-infrared), and ultraviolet wavelengths.

The multilayer stack 204 is formed over a front side 202 f of thesubstrate 202. The multilayer stack 204 serves as a radiation-reflectivelayer of the photomask 200. The multilayer stack 204 may include pairswherein each pair is formed of a molybdenum (Mo) layer and a silicon(Si) layer. The number of alternating Mo layers and Si layers (i.e., thenumber of Mo/Si pairs) and the thicknesses of the Mo layers and the Silayers are determined so as to facilitate constructive interference ofindividual reflected rays (i.e., Bragg reflection) and thus increase thereflectivity of the multilayer stack 204.

The capping layer 206 is disposed over the multilayer stack 204. In someembodiments, the capping layer 206 is used to prevent oxidation of themultilayer stack 204 during a mask patterning process. In someembodiments, the capping layer 206 is made of ruthenium (Ru) orruthenium oxide (RuO₂). Other capping layer materials, such as silicondioxide (SiO₂), amorphous carbon or other suitable compositions, canalso be used in the capping layer 206.

The light-absorption layer 208 is disposed over the capping layer 206.In some embodiments, the light-absorption layer 208 is ananti-reflective layer that blocks or absorbs radiation in EUV wavelengthranges impinging onto the photomask 200. The light-absorption layer 208may include chromium, chromium oxide, titanium nitride, tantalumnitride, tantalum oxide, tantalum boron nitride, tantalum, titanium,aluminum-copper, combinations thereof, or the like. The light-absorptionlayer 208 may be formed of a single layer or of multiple layers. Forexample, the light-absorption layer 208 includes a chromium layer and atantalum nitride layer.

In some embodiments, an antireflective layer (not shown) is disposedover the light-absorption layer 208. The antireflective layer may reducereflection of the impinging radiation having a wavelength shorter thanthose of the MTV range from the light-absorption layer 208, and mayinclude a same pattern as that of the underlying light-absorption layer208. Other materials, such as Cr₂O₃, ITO, SiN and TaO₅, may also beused. In other embodiments, a silicon oxide film is adopted as theantireflective layer.

In some embodiments, the photomask 200 further includes a conductivelayer 212 on a backside of the substrate 202. The conductive layer 212may aid in engaging the photomask 200 with an electric chuckingmechanism (not separately shown) in a lithography system. In someembodiments, the conductive layer 212 includes chromium nitride (CrN),chromium oxynitride (CrON), or another suitable conductive material.

Referring to FIGS. 1 and 2 , a first type defect of the photomask 200may be present in a form of a contamination particle P1 residing on thesurface of the photomask 200, e.g., the capping layer 206 or theabsorption layer 208. In some embodiments, the contamination particle P1may obscure the illumination and reflection of the radiation beam of themultilayer stack 204. In an example, a phase-defected portion P2 of themultilayer stack 204 is present as a second type defect, which wouldcause phase errors in the reflected radiation beam. In a photomaskinspection operation, the inspection apparatus 100 is operated todetermine whether any defects exist by identifying the defect P1 or P2in the inspection image. The defected areas of the inspection imagecorresponding to the defects P1 and P2 of the photomask 200 may be shownwith different grayscales as compared to their neighboring features dueto scattered radiation beam R3 around the defect P1 or P2. Therefore,defect detection effectiveness is determined by the characteristics ofthe reflected radiation beam R3, e.g., the illumination intensity, theintensity variation and illumination symmetry in different axes.

FIG. 3A is a schematic top view of the POB 120, in accordance with someembodiments of the present disclosure. Some features of the POB 120,such as the reflective elements 124 and 126, are omitted from FIG. 3Afor clarity. As discussed above, the aperture stop 122 is configured tofilter undesired portions of the radiation beam R3 through the aperture122A for obtaining a better-quality inspection image. In someembodiments, the aperture 122A is designed to increase the illuminousflux entering the POB 120 so as to enhance ability to detect defects. Inaddition, the geometry of the aperture 122A, such as its location andshape, is related to the chief ray of the radiation beam R3 as shown inFIG. 1 to capture a maximal amount of the radiation beam R3 with asymmetrical optical distribution. In some embodiments, the performanceof the aperture stop 122 is compromised by various structurallimitations of the POB 120, and thus the existing aperture cannotachieve a theoretically optimal design to allow for a maximal amount oflight to pass through. The structure limitations of the POB 120 include,but are not limited to, the arrangement of the holder 128 and the secondreflective element 126.

In some embodiments, the aperture stop 122 is defined by a periphery orcircumference 302. In other words, periphery 302 is shared by theaperture stop 122 and the ring of the holder 128. In some embodiments,the aperture 122A is arranged within the area defined by the periphery302 or the holder 128. In the aperture stop 122, the aperture 122A isarranged on one side of the aperture stop 122. In some embodiments, theaperture 122A contacts the ring of the holder 128. In some embodiments,the aperture stop 122 has a circular shape with a center C1, and theaperture 122A has a circular shape with a center C2, wherein the centerC2 does not coincide with the center C1 of the aperture stop 122. Insome embodiments, the aperture 122A and the aperture stop 122 are notconcentric.

In some embodiments, the aperture stop 122 has a half-width D1 measuredbetween the center C1 and the periphery 302 of the holder 128. In caseswhere the aperture stop 122 has a circular shape, the half-width D1 is aradius of the aperture stop 122. In embodiments where the aperture 122Ahas a circular shape, the aperture 122A has a diameter D2 substantiallyequal to the radius D1. In some embodiments, the aperture 122A contactsor is tangent to the holder 128 at a point T1. In some embodiments, theaperture 122A contacts or is tangent to the periphery of the aperturestop 122 at a point T1. The aperture 122A may also contact or be tangentto the center C1. In the present embodiment, a distance D5 between thecenter C2 and the point T1 is equal to the distance D3.

In the proposed photomask inspection scheme, the aperture 122A isarranged to have a symmetric shape. As discussed above, when theaperture 122A has a circular shape, the aperture 122A is symmetricalwith respect to any axis that extends through the center C2 of theaperture 122A. In some embodiments, the aperture 122A is arranged to besymmetrical with respect to at least two axes X1 and X2, wherein theaxes X1 and X2 are perpendicular to each other.

Through the proposed aperture 122A, higher order diffraction of theradiation beam R3 is blocked by the aperture stop 122, and theilluminous flux of the radiation beam R3 can achieve a maximal valueunder the constraint of the holder 128 while the optical distribution ofthe radiation beam R3 is made to be as uniform as possible due to thesymmetry of the aperture 122A.

FIG. 3B is a schematic view of the illumination radiation beam R2 andthe reflected radiation beam R3, in accordance with some embodiments ofthe present disclosure. Referring to FIGS. 3A and 3B, the radiation beamR2 is incident on the photomask 111 at a tilt angle θ, wherein the tiltangle θ is measured between a chief ray K2 of the radiation beam R2 andan axis N1, which is a normal line perpendicular to the surface of thephotomask 111, The radiation beam R3 also has a tilt angle θ measuredbetween the chief ray K3 of the radiation beam R3 and the axis N1 due tothe principle of reflection. Referring to FIG. 1 and FIG. 3B, theradiation beam R2 with the tilt angle θ may be obtained by controllingthe mirror 108, e.g., by adjusting a tilt angle β of the mirror 108and/or shifting the location of the mirror 108 according to the tiltangle θ.

In some embodiments, the radiation beam R2 has a marginal ray to definethe range of a beam cone, in which a half-angle γ of the beam conerepresents a convergence or divergence of the radiation beam R2. In someembodiments, the light within the beam cone of radiation beam R2includes an intensity level of about 1/e² (i.e., about 13.5%) of thetotal intensity of the radiation beam R2. In some embodiments, the angleγ is set at about six degrees. In some other embodiments, other valuesof the angle γ greater than or less than six degrees, e.g., fivedegrees, are also possible. In some embodiments, the angle γ isdetermined according to an existing tilt angle α used in a lithographyoperation. In some embodiments, the existing tilt angle α is differentfrom the proposed tilt angle θ.

In some embodiments, the tilt angle θ is determined according to theconfigurations of the aperture 122A and the aperture stop 122 foraligning the chief ray K3 of the radiation beam R3 with the center C2 ofthe aperture 122A. In some embodiments, the tilt angle θ is determinedbased on the location of the center C2 of the aperture 122A. In someembodiments, the tilt angle θ is calculated by the following formula:θ=tan⁻¹(D3/H1),

wherein the width D3 denotes a horizontal distance between the center C2of the aperture 122A and the center of an illumination spot S1 of thephotomask 111 at which the radiation beam R2 impinges onto thephotomask. The height H1 represents the vertical distance between thesurface of the photomask 111 (or the stage 110) and the aperture stop122. In some embodiments, the center C1 of the aperture stop 122vertically coincides with the illumination spot S1 of the photomask 111,and thus the width D3 is equal to the radius D3 of the aperture 122Ashown in FIG. 3A given that the aperture 122A has a circular shape.

In some embodiments, the tilt angle θ is set at greater than the angleγ. In some embodiments, the tilt angle θ is set at greater than theangle γ by about three degrees. In some embodiments, the tilt angle θ isin a range between about 8.5 degrees and about 9.5 degrees, or betweenabout 8.8 degrees and about 9.2 degrees. In some embodiments, the tiltangle θ is set at about nine degrees.

In some embodiments, a numerical aperture (NA) associated with theradiation beam R2 is defined by the following equation,NA=sin⁻¹(π/180*θ).

Through the proposed design of the aperture 122A combined with theoperation of the radiation beam R2 having the incident tilt angle θ, theamount of light entering the detector 130 can be increased while thespatial distribution of the radiation beam R2 is made more uniform,thereby mitigating or eliminating feature distortion in the inspectionimage.

In some embodiments, the size or width of the aperture 122A isdetermined according to the location of the second reflective element126 of the POB 120. In some embodiments, referring to FIG. 1 and FIG.3B, the radiation beam R3 is designed to travel from the photomask 111to the first reflective element 124. Therefore, the radiation beam R3should be prevented from hitting the second reflective element 126before reaching the first reflective element 124. In some embodiments,the aperture 122A is laterally spaced apart from the second reflectiveelement 126 in order to ensure that the radiation beam R3 is not blockedby the second reflective element 126. In some embodiments, the (right)side of the aperture 122A is defined by the (left) side of the secondreflective element 126 such that the aperture 122A does not overlap thesecond reflective element 126 from a top-view perspective. In someembodiments, since the second reflective element 126 is arranged at thecenter C2 and coincides with the center C1 of the aperture stop 122 froma top-view perspective, the maximal value of the diameter D2 of theaperture 122A is limited by the center C1.

In some embodiments, as illustrated in FIG. 1 and FIG. 3B, the aperture122A is designed to allow the received radiation beam R5 to pass throughthe POB 120 and reach the detector 130. Therefore, the aperture 122A hasat least a portion overlapping the second reflective element 126 whenviewed from above such that the received radiation beam R5 can propagatethrough the POB 120 without being blocked.

FIG. 4A is a schematic top view of the POB 120, in accordance withanother embodiment of the present disclosure. The POB 120 shown in FIG.4A is similar to the POB 120 shown in FIG. 3A, except that the aperturestop 122 shown in FIG. 4A includes an aperture 122B with a shapedifferent from that of the aperture 122A. In some embodiments, theaperture 122B has an elliptical shape, wherein the ellipse has a majoraxis that extends in the direction of the axis X2 and a minor axis thatextends in the direction of the axis X1. Although the aperture 122B hasa shape different from that of the aperture 122A, the aperture 1223 isstill within the scope of the aperture stop 122 as defined by the holder128 or the periphery 302.

The ellipse of the aperture 122B has a width D2, i.e., a dimension ofthe minor axis measured from a point T1 that contacts the holder 128 tothe center C1 of the aperture stop 122. A center C2 of the aperture 122Bis located at the intersection of the major and minor axes of theaperture 122B. The width D3 of the semi-minor axis of the aperture 122Bis equal to the radius D3 of the aperture 122A. In the presentembodiment, the distance D5 of the aperture 122B between the center C2and the point T1 is equal to the distance D3 of the aperture 122B.

The aperture 122B is symmetrical with respect to the axes X1 and X2. Assuch, the aperture 122B is configured to seek a balance betweenmaximizing its area and maintaining symmetry of its shape. According tothe arrangement of the aperture 122B, the area of the aperture 122B isgreater than the area of the aperture 122A, and thus the total amount oflight passing through the aperture 122B is greater than that passingthrough the aperture 122A while still maintaining the advantage ofsymmetry with respect to the axes of the major and minor axes.

FIG. 4B is a schematic top view of the POB 120, in accordance with yetanother embodiment of the present disclosure. The POB 120 shown in FIG.4B is similar to the POB 120 shown in FIG. 3A, except that the aperturestop 122 shown in FIG. 4B includes an aperture 122C with a polygonalshape, such as a rectangle, a hexagon, an octagon, or the like. In someembodiments, the aperture 122C has a hexagonal shape with three verticescoinciding with the holder 128 or the periphery 302 of the aperture stop122. Although the aperture 122C has a shape different from that of theaperture 122A or 122B, the aperture 122C is still within the scope ofthe aperture stop 122 as defined by the holder 128.

The polygon of the aperture 122C is symmetrical with respect to the axesX1 and X2. The symmetry of the aperture 122C with respect to at leasttwo axes provides reduced image distortion while allowing a sufficientamount of light to pass through.

FIG. 4C is a schematic top view of the POB 120, in accordance with yetanother embodiment of the present disclosure. The POB 120 shown in FIG.4C is similar to the POB 120 shown in FIG. 3A, except that the aperturestop 122 shown in FIG. 4B includes an aperture 122D, in which a portionE1, which overlaps the center C1, is cut off from the circular shape ofthe aperture 122A in FIG. 3A to facilitate imaging performance of theinspection apparatus 100. In other words, the aperture stop 122,including the cut-off portion E1, defines the shape of the aperture122D, In some embodiments, the diameter of the aperture 122D is greaterthan the diameter D2 of the aperture 122A, and the cut-off portion E1 isshifted to the right side of the center C1 and offset from the centerC1, In such circumstance, the aperture 122D overlaps the center C1 froma top-view perspective. In some embodiments, the aperture 122D overlapsthe second reflective element 126 from a top-view perspective.

FIG. 5 is a schematic diagram of a lithography apparatus 500, inaccordance with some embodiments of the present disclosure. Thelithography apparatus 500 may be configured to perform reflective typelithography, such as extreme ultraviolet (EUV) lithography, ortransmission type lithography. In some embodiments, the lithographyapparatus 500 is configured to perform lithography operation using areflective type photomask. The lithography apparatus 500 of FIG. 1includes a radiation source 502, an illuminator 504, a mask stage 506, aprojection optics box (POB) 510, and a substrate stage 518.

The radiation source 502 is configured to generate a source radiationbeam S0, e.g., an EUV light having a wavelength between about 1 nm andabout 100 nm, such as 13.5 nm. In some embodiments, the radiation source502 is similar to the radiation source 102 of the inspection apparatus100.

In some embodiments, the illuminator 504 includes reflective optics,such as one or more mirrors, to direct light from the radiation source502 through one or more reflections to form illumination radiation beamsS1 and S2, in which the illumination radiation beam S2 impinges onto themask stage 506. In some examples, the illuminator 504 includes a zoneplate to improve focus of the source radiation beam S0. In someembodiments, the illuminator 504 is configured to shape the sourceradiation beam S0. In some embodiments, the illuminator 504 isconfigured to provide an on-axis illumination (ONI) to the photomask508. In some embodiments, the illuminator 504 is configured to providean off-axis illumination (OAI) to the photomask 508. In someembodiments, the illuminator 504 has a configuration similar to that ofthe illuminator 106 of the inspection apparatus 100.

The mask stage 506 is configured to secure a photomask 508, wherein thephotomask 508 is similar to the photomask 111 or 200 as discussed above.In some embodiments, the mask stage 506 includes an electrostatic chuck(e-chuck) to secure the photomask 508.

The wafer stage 518 is used for supporting and holding a workpiece 516.The wafer stage 518 may include one or more positioning devices, such asmotors and roller guides, to provide accurate alignment and movement ofthe workpiece in various directions for achieving better performance infocusing, leveling, exposure or other movements.

The workpiece 516 is provided with a substrate having a material layerformed thereon. The substrate may be a wafer substrate. In variousembodiments, the wafer substrate includes a semiconductor wafer, such asa silicon wafer, germanium wafer, silicon-germanium wafer, III-Vsemiconductor wafer, or other type of wafer as known in the art. Thematerial layer may be a photosensitive material, e.g., a photoresistsensitive to EUV radiation.

A reflected radiation beam S3 is reflected from the photomask 508 anddirected toward the POB 510. The POB 510 serves the functions oftransferring the image of the circuit pattern on the photomask 508 tothe workpiece 516. The POB 510 may be configured to focus theillumination radiation beam S3 and projects a projection radiation beamS4 onto the workpiece 516. The projection radiation beam S4 is used forpatterning the workpiece 516 by transferring the circuit pattern of thephotomask 508 onto the workpiece 516 through the POB 510. The POB 510may include one or more reflective optics for forming the projectionradiation beam S4. In some embodiments, the configurations of the POB510 of the lithography apparatus 500 and the POB 120 of the inspectionapparatus 100 may be similar or different.

In some embodiments, the optical components of the illuminator 504 areconfigured to provide an illumination to the photomask 508 atpredetermined locations. The optical components of the illuminator 504are configurable to reflect the illumination radiation beam S2 toimpinge on the photomask 508 at a predetermined tilt angle α. The tiltangle α is determined to increase reflectivity of the photomask 508while reducing a shadow effect. In some embodiments, the tilt angle α isdetermined according to the optical configurations of the illuminator504 and/or the POB 510 to achieve desirable lithography performance ofthe workpiece 516. In some embodiments, the tilt angle α is in a rangebetween about 5.0 degrees and about 7.0 degrees, such as 6.0 degrees.

In some embodiments, the tilt angle θ is set at greater than the tiltangle α. In some embodiments, the tilt angle θ is set at greater thanthe tilt angle α by about three degrees.

Since the configurations of the illuminator 504 and the POB 510 may notbe similar to the configurations of the illuminator 106 and the POB 120,operating the inspection apparatus 100 by setting the tilt angle θ to beidentical to the tilt angle α of the lithography apparatus 500 may notprovide the optimal imaging performance. Further, the existing apertureshape on the aperture stop 122 shown in FIG. 1 may not be symmetricaldue to the constraint of the tilt angle α. In contrast, the proposedaperture design is not limited by the constraint of the tilt angle αdetermined for the lithography apparatus 500 for pursuing the maximallight along with a symmetrical aperture shape. The tilt angle of theradiation beam for illuminating the photomask is also determinedaccording to the new design of the symmetrical aperture. As a result,the inspection image generated by the proposed scheme can provide betterintensity performance and reduced edge distortion, e.g., improving theblurred edges of features or scale asymmetry between the features in theX-axis and Y-axis.

FIG. 6 is a flowchart of a method 600 of inspecting a photomask, inaccordance with some embodiments of the present disclosure. It should beunderstood that additional steps can be provided before, during, andafter the steps shown in FIG. 6 , and some of the steps described belowcan be replaced or eliminated, for additional embodiments of the method.The order of the steps may be changed. Materials, configurations,dimensions, processes and/or operations the same as or similar to thosedescribed with respect to the foregoing embodiments may be employed inthe following embodiments, and the detailed explanation thereof may beomitted.

At step 602, a photomask is received. The photomask may be the photomask111, 200 or 508 discussed above. At step 604, a wafer is patterned bydirecting a first radiation beam to the wafer through the photomask at afirst tilt angle α. The wafer is patterned by a lithography operationperformed by a lithography apparatus, such as the lithography apparatus500. The wafer may be similar to the workpiece 516 shown in FIG. 5 . Insome embodiments, the first tilt angle α is set at six degrees.

At step 606, an inspection apparatus, e.g., the inspection apparatus 100shown in FIG. 1 , is operated to inspect the photomask. The inspectionapparatus has a POB including an aperture stop. The aperture stop mayinclude an aperture with a symmetrical shape, as discussed withreference to FIGS. 3A, 3B, 4A and 4B. The operation of the inspectionapparatus may include steps 608, 610, 612 and 614, as described below.

At step 608, a second tilt angle θ of a second radiation beam isdetermined according to an aperture of the aperture stop. In someembodiments, the second tilt angle θ is greater than the first tiltangle α.

At step 610, the second radiation beam is directed to the photomask atthe second tilt angle θ. In some embodiments, the operation of theinspection apparatus includes rotating or shifting a mirror of theinspection apparatus to cause the second radiation beam to directed tothe photomask at the second tilt angle θ.

At step 612, a third radiation beam reflected from the photomask isreceived through the aperture. At step 614, an image of the photomask isgenerated according to the third radiation beam.

In some embodiments of the present disclosure, after the inspection ofthe photomask, the method 600 returns to step 604 to perform thelithography operation of the wafer or another wafer if no defects arefound. In some other embodiments, a repairing operation is performed tofix the defect of the photomask before the photomask is sent for anotherlithography operation. The order of steps 604 and 606 may beinterchanged.

According to an embodiment of the present disclosure, a method includes:receiving a photomask; patterning a wafer by directing a first radiationbeam to the wafer through the photomask at a first tilt angle; andinspecting the photomask. The inspecting includes: directing a secondradiation beam to the photomask at a second tilt angle greater than thefirst tilt angle; receiving a third radiation beam reflected from thephotomask; and generating an image of the photomask according to thethird radiation beam.

According to an embodiment of the present disclosure, a method includes:operating an inspection apparatus to inspect a photomask, the inspectionapparatus including an aperture stop, wherein the aperture stop has afirst width measured from a periphery of the aperture stop to a centerof the aperture stop; determining a tilt angle of a first radiation beamaccording to the first width, wherein the tilt angle is measured betweena chief ray of the first radiation beam and a first axis perpendicularto a surface of the photomask; directing the first radiation beam to thephotomask at the tilt angle; and receiving a second radiation beamreflected from the photomask through an aperture of the aperture stop.

According to an embodiment of the present disclosure, a method includes:operating an inspection apparatus to inspect a photomask, the inspectionapparatus having a projection optics box including an aperture stop, andthe aperture stop defining a circular aperture coinciding with a centerof the aperture stop; determining a first tilt angle according to theaperture, the first tilt angle being greater than about six degrees;directing a first radiation beam onto the photomask at the first tiltangle; and receiving, by the projection optics box, a second radiationbeam reflected from the photomask through the aperture, wherein thesecond radiation beam has a chief ray passing through a center of theaperture.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure, Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method, comprising: receiving a photomask;patterning a wafer by directing a first radiation beam to the waferthrough the photomask at a first tilt angle; and inspecting thephotomask by an inspection apparatus comprising an aperture stop, theinspecting comprising: directing a second radiation beam to thephotomask at a second tilt angle greater than the first tilt angle;receiving a third radiation beam reflected from the photomask through anaperture of the aperture stop, wherein the aperture is tangent at acenter of the aperture stop; and generating an image of the photomaskaccording to the third radiation beam.
 2. The method according to claim1, wherein the first radiation beam and the second radiation beam have awavelength of about 13.5 nm.
 3. The method according to claim 1, whereinthe second tilt angle is greater than the first tilt angle by aboutthree degrees.
 4. The method according to claim 1, wherein the secondtilt angle is about nine degrees.
 5. The method according to claim 1,wherein directing a second radiation beam to the photomask comprisesconfiguring a third tilt angle of a mirror such that the secondradiation beam is reflected by the mirror and impinges on the photomaskat the second tilt angle.
 6. The method according to claim 1, wherein achief ray of the third radiation beam coincides with a center of theaperture.
 7. The method according to claim 1, wherein the aperture issymmetrical with respect at least to a first axis and a second axisdifferent from the first axis.
 8. The method according to claim 1,wherein the aperture has a diameter substantially equal to a radius ofthe aperture stop.
 9. The method according to claim 1, further comprisesa holder configured to hold the aperture stop, wherein the aperture istangent to the holder.
 10. The method according to claim 9, wherein theholder laterally surrounds the aperture stop and has a circular shape.11. A method, comprising: operating an inspection apparatus to inspect aphotomask, the inspection apparatus comprising an aperture stop, whereinthe aperture stop has a first width measured from a periphery of theaperture stop to a center of the aperture stop; determining a tilt angleof a first radiation beam according to the first width, wherein the tiltangle is measured between a chief ray of the first radiation beam and afirst axis perpendicular to a surface of the photomask; directing thefirst radiation beam to the photomask at the tilt angle; and receiving asecond radiation beam reflected from the photomask through an apertureof the aperture stop, the aperture tangent at the center of the aperturestop.
 12. The method according to claim 11, wherein the first radiationbeam is formed as a beam cone having an angle less than the tilt angle.13. The method according to claim 11, wherein the aperture is tangent atthe periphery of the aperture stop.
 14. The method according to claim11, wherein the aperture is symmetrical with respect to a second axisperpendicular to a third axis along which the first width is measured.15. The method according to claim 11, wherein the aperture has acircular shape.
 16. The method according to claim 11, wherein theinspection apparatus further includes a reflective element configured toreceive the second radiation beam, wherein the first width of theaperture is further determined according to a location of the reflectiveelement.
 17. A method, comprising: operating an inspection apparatus toinspect a photomask, the inspection apparatus comprising a projectionoptics box including an aperture stop, and the aperture stop defining anaperture, wherein the aperture is circular and tangent at a center ofthe aperture stop; determining a first tilt angle according to theaperture, the first tilt angle being greater than about six degrees;directing a first radiation beam onto the photomask at the first tiltangle; and receiving, by the projection optics box, a second radiationbeam reflected directly from the photomask through the aperture, whereinthe second radiation beam has a chief ray passing through a center ofthe aperture.
 18. The method according to claim 17, wherein theinspection apparatus comprises a mirror for reflecting a third radiationbeam to form the first radiation beam, wherein directing a radiationbeam onto the photomask at the first tilt angle comprises controlling asecond tilt angle of the mirror according to the first tilt angle. 19.The method according to claim 17, wherein a first distance between thecenter of the aperture and the center of the aperture stop is equal to asecond distance between the center of the aperture and a periphery ofthe aperture stop.
 20. The method according to claim 17, wherein theaperture contacts a periphery of the aperture stop.